The meteor that exploded over the Russian city of Chelyabinsk without warning Friday (Feb. 15), damaging hundreds of buildings and wounding more than 1,000 people, was caused by a space rock about 50 feet (15 meters) wide, researchers said.

Asteroids of this size are both difficult to detect and incredibly numerous, so it will take a long time for astronomers to find and map out the orbits of all the potentially dangerous ones. Besides, researchers have bigger fish to fry.

"Defending the Earth against tiny asteroids such as the one that passed over Siberia and impacted there is a challenging issue that is something that is not currently our goal," Paul Chodas, a scientist with the Near Earth Object Program Office at NASA's Jet Propulsion Laboratory in Pasadena, Calif., told reporters Friday (Feb. 15).

"We are focusing on the larger asteroids first," Chodas added. "They are the ones that are the most hazardous."

In 1998, Congress directed NASA to find all of the near-Earth asteroids at least 0.6 miles (1 kilometer) wide that could pose an impact risk to Earth. Such large space rocks have the potential to end human civilization if they hit us.

NASA met that challenge several years ago, and its scientists have now identified 95 percent of the 980 such mountain-size asteroids thought to be cruising through Earth's neighborhood.

Observations by NASA's WISE space telescope, for example, suggest that about 4,700 asteroids at least 330 feet (100 m) wide come uncomfortably close to our planet at some point in their orbits.